1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and more specifically to a bipolar transistor having an emitter region formed of a semiconductor having a large forbidden band width, such as a silicon carbide (indicated as "SiC" including SiCx expressed in a general chemical formula).
2. Description of Related Art
Referring to FIG. 1, there is shown a diagram illustrating a relation between respective regions and a depletion layer in a bipolar transistor disclosed in Japanese Patent Application Laid-open Publication No. JP-A-5-243253. The bipolar transistor shown in FIG. 1 includes emitter region 31, a base region 32 and a collector region 33 arranged in the named order in such a manner that a PN junction is formed between the emitter region 31 and the base region 32 and between the base region 32 and the collector region 33. Each of the base region 32 and the collector region 33 is formed of silicon (Si), and the emitter region 31 is formed of SiC 36, so that a Si type HBT (heterobipolar transistor or heterojunction bipolar transistor).
In operation, under a given biasing condition, a depletion layer 34 is formed between the emitter region 31 and the base region 32, because of the PN function between the emitter region 31 and the base region 32, and a depletion layer 35 is formed between the base region 32 and the collector region 33, because of the PN junction between the base region and the collector region 33.
The reason for using the emitter formed of SiC is that, as explained in JP-A-5-243253, even if an impurity concentration of the base region is made high in order to make a base resistance small, an increase of a tunnel current can be suppressed since a forbidden band of the emitter is wide.
In this connection, the thicker the SiC layer is, the more it is effective in suppressing the increase of the tunnel current. However, if the SiC layer is too thick, an emitter resistance becomes high. The cause for this is that it is difficult to activate the SiC layer by adding impurities into SiC with a high concentration.